A master cylinder is a device for generating hydraulic power in a hydraulic brake system.
A master cylinder includes a cylinder housing and a piston which is movably disposed in the cylinder housing. Further, a sealing member which is installed in an inner surface of the housing is provided, and the piston is provided with a communication hole. The sealing member has a cup shape and is also called a cup seal.
The cylinder housing and the piston forms cooperatively a pressure chamber for generating hydraulic pressure in a side in a direction of forward movement of the piston, and a hydraulic supplying chamber which communicates with a reservoir is formed between the cylinder housing and an outer surface of the piston. The sealing member partitions the hydraulic supplying chamber and the pressure chamber.
In a prior art, a communication hole for connecting the pressure chamber and the hydraulic supplying chamber is formed in the piston, and communications holes are respectively formed at a front side and a rear side, the front communication hole acting as a passageway for pressure generation and the rear communication hole acting as a passageway for preventing drag. At this time, an indentation portion is formed at an outer circumferential surface of the piston, and a front wall and a rear wall of the indentation portion are respectively formed as a slanted surface, a portion between the front wall and the rear wall being formed in a planar surface parallel with a longitudinal direction of the cylinder, and the two communication holes being formed in the planar surface. In this case, there is a problem in that the sealing member may contact a portion which forms an entrance of a passageway so as to be damaged.